The aquatic vegetation of the Upper Danube river : past and present

We compare historical and contemporary data of the flora and vegetation of the Upper Danube river, two reservoirs, and the 20 largest oxbows between the confluence of the headwaters Brigach and Breg and the influx of the Iller (Baden-Wurttemberg, Germany) based on two recent surveys (1989 and 2004) and the evaluation of old herbaria material, regional botanical literature and old floras. In 2004, the Upper Danube River accommodated 28 vascular macrophytes and eight moss species below the low water level. The contemporary aquatic vegetation is characterized by the dominance of Ranunculus fluitans and the sparse occurrence of species sensitive to eutrophication. Nuphar lutea is the dominating species in the oxbows and reservoirs, but also in many slow flowing stretches of the river. The most striking phenomenon observed in all water bodies is a decline or extinction of broad-leaved pondweeds. In the main channel, a loss of Potamogeton-species and Hippuris vulgaris was counterbalanced by the invasion of eutraphent, prevailingly neophytic species (Elodea spp.). A decline in species richness and a change in community composition occurred to a lesser extent after channelization carried out mainly in the early 20 th century and after extensive eutrophication commencing in the 1950s. Historical floristic records enabled a description of reference communities for the Upper Danube as required by the Water Framework Directive (WFD). Oxbows and reservoirs accommodated a more diverse flora than the river channel, but many species were rare. Historical data allowed us to trace back the temporal succession of vegetation in one case. Recent rehabilitation measures led to a rapid increase in species diversity and vegetation cover in several river stretches owing to colonization of abundant species with a high vegetative dispersal capacity. We conclude that the re-assembly of former aquatic communities will fail owing lo the contemporary paucity or extinction of species sensitive to eutrophication and disturbances even when past habitats are restored.